1. Field of the Invention
This invention resides in the field of scanning systems for arrays of biological samples, particularly where each sample contains two or more targets that are individually labeled.
2. Description of the Prior Art
Detection systems that detect multiple species in each of a large number of samples or reaction mixtures are typified by the thermal cyclers that are used in performing the polymerase chain reaction (PCR). In PCR and other such systems, the samples or reaction mixtures are typically distributed among individual wells arranged in two-dimensional arrays, such as in a conventional microtiter plate, and monitoring of the sample conditions and the progress of each reaction requires separate detection and quantitation of each of the target species in each well. Such monitoring is commonly achieved by the attachment of fluorescent labels to the target species with a distinct label bound to each target, and detection and quantitation of the targets are achieved by optical scanning in which each label receives excitation light at a wavelength band appropriate to the label and the emission light resulting from each excited label is separately detected. Since the excitation bands of different fluorophores are often close together and frequently overlap, as do the emission bands, common scanning systems contain a separate optical system for each fluorophore and thus each target. A six-color system, for example, will contain six separate optical channels in the scanning head, each channel containing its own light source (typically a light-emitting diode, or LED), its own excitation filter, its own dichroic mirror, its own emission filter, its own set of lenses, and its own detector. The scanning head is then aligned with six wells at once, with a single optical channel aimed at each well, and the head is driven across the well array in one-well increments along both axes so that each well is ultimately exposed to all six optical systems. The number of course will vary with the number of targets, and hence the number of colors. In all cases, however, the scanning head suffers from a high part count, bulky construction, and high cost.